This is the most revealing PPO video that I've seen.
Of interest:
The craft has a H-stab but its volume is insufficient.
The pilot porpoises a little during his fast low pass.
The airspeed at the top of the zoom climb appears not to be great.
At the top of the zoom, the pilot pushes the stick forward, but soon yanks it back. The rotor follows the aft stick command but this has no effect on the developing pitchover.
Raghu's observation that a downdraft should not lead to a PPO in a gyro with an AOA-stable airframe is correct, with certain assumptions: (1) the H-stab is of sufficient volume (area x moment arm) and receives airflow unobstructed by other components (2) the rotor spindle stays at the same angle to the frame throughout the downdraft encounter (i.e the pilot does not "float" the stick, nor does he push it forward because he has gotten behind the aircraft), and (3) RRPM loss during the event is not great.
I like immersion of a downloaded H-stab in a HTL gyro because (if properly designed) it can more completely compensate for the pitching effect of the HTL when forward stick is applied. If the H-stab is not downloaded and/or is outside the propwash and airspeed is modest, then, when the stick is pushed forward, the nose of the craft will drop, amplifying the effect of the control input until the H-stab achieves enough of a negative AOA to arrest further nose-down rotation. Obviously, if the H-stab lacks sufficient potency, it will NEVER reach an AOA at which it arrests the PPO -- instead it will stall and the result may well be as seen on the video. A number of gyros WITH h-stabs have PPOed -- though AFAIK they've had either very small H-stabs (Bensens, etc.) or they have occuured in gyros with large engines, large HTL offsets and modest H-stab volume.
All of this talk concerns pitch stability only. Roll stability is another topic. Fortunately, roll stability can be achieved not only with immersed H-stabs, but just as well with immersed vertical fins, or immersed fins at angles between vertical and horizontal. Immersing fins for this purpose is especially handy, in that, if an anti-torque incidence is built into them, the counter-torque that they continuously apply to the frame will vary directly with throttle setting (or more precisely, with slipstream speed). The counter-torque of a non-immersed fin will vary with aircraft airspeed -- not always a good proxy for throttle setting.